'''
给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

 

示例 1：


输入：root = [1,null,2,3]
输出：[1,2,3]
示例 2：

输入：root = []
输出：[]
示例 3：

输入：root = [1]
输出：[1]
示例 4：


输入：root = [1,2]
输出：[1,2]
示例 5：


输入：root = [1,null,2]
输出：[1,2]

'''
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        result = []
        def prevtra(root):
            if root == None:
                return
            else:
                result.append(root.val)
                prevtra(root.left)
                prevtra(root.right)
        prevtra(root)
        return result
#迭代
class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        result = []
        stack = []
        if root == None:
            return result
        stack.append(root)
        while len(stack) > 0:
            tmp = stack.pop()
            result.append(tmp.val)
            if tmp.right != None:
                stack.append(tmp.right)
            if tmp.left != None:
                stack.append(tmp.left)
        return result